1. Field of the Invention
The present invention relates to an energy curable inkjet ink with a stray light resistance of at least 7, and a method to increase the stray light resistance of an energy curable inkjet ink to at least 7.
2. Discussion of the Related Art
Inkjet technology can deposit materials with different chemical and physical properties. Inkjet finds applications in many graphics applications including point of purchase, vehicle wraps, wide format printing, and it has also been used in electronics including the manufacture of solar panels and PCBs.
Energy curable inkjet inks are susceptible to build up of cured material at the inkjet nozzle. Traditional stabilizers do not sufficiently address the susceptibility of energy curable inkjet inks to stray light, or if they do, cure is compromised. This can be a problem in LED UV cured inks as sensitivity to UV in the UVA region means they can be susceptible to stray light.
A typical drop-on-demand (DoD) inkjet printhead may include several ink channels in parallel. Each channel has a piezo-actuator, which on application of a standard actuation voltage pulse, generates pressure oscillations inside the ink channel. These pressure oscillations then push the ink drop out of the nozzle. The print quality delivered by an inkjet printhead depends on the properties of the jetted drop. The following drop properties are preferred to give acceptable image quality, reliability and printhead performance:
Drop Velocity
The drops preferably need to be ejected at a consistent velocity to reduce dot positioning errors. If the drop speed is too slow, the sensitivity to variation in printhead to substrate distances will be higher and result in a reduction in image quality. This sensitivity is reduced if the time of flight is reduced.
Drop Mass/Volume
Depending on the application, the drop volume requirements will vary considerably with some applications needing the drop volume to be varied during the application. For the coverage of larger areas, larger drops are needed. While for high resolution printing, smaller drops are required.
Drop Shape
The formation of tails or satellite drops will negatively influence the shape of a drop on a substrate. These are not desirable for image quality.
Jet Straightness
The drops preferably need to be ejected in a straight line towards the substrate. Typically within 10 mrad accuracy or a reduction in image quality will be seen.
FIG. 1 is a cross sectional view of an ink channel with no drop deviation. FIG. 2 is a cross sectional view of an ink channel with drop deviation due to a partially obstructed nozzle. FIG. 3 shows normal operation of a printhead with no nozzle deviation. FIG. 4 shows operation of a printhead with nozzle deviation. In FIG. 4, the main drop has merged with the adjacent drop with a small satellite drop thrown to one side.
The build-up of cured material at the nozzle affects jet straightness and thus printing quality. Furthermore, once jet deviation has occurred, if it can't be recovered by flushing, then the printhead will need to be replaced.
Thus there is a need to reduce the build up of cured ink caused by exposure to low levels of UV light from stray light sources and thereby to improve printing quality and extend the life of the printheads.